/Engine/Plugins/Online/OnlineSubsystem/Source/Public/Interfaces/OnlineExternalUIInterface.h
Display the platform UI for sending a mailbox message to the specified user. Will attempt to automatically send the message if the platform allows for it.
true if it was able to send the message and show the UI, false if it failed
virtual bool ShowSendMessageToUserUI ( int32 LocalUserNum, const [FUniqueNetId](API\Runtime\CoreOnline\Online\FUniqueNetId) & Recipient, const [FShowSendMessageParams](API\Plugins\OnlineSubsystem\Interfaces\FShowSendMessageParams) & ShowParams, const [FOnShowSendMessageUIClosedDelegate](API\Plugins\OnlineSubsystem\Interfaces\FOnShowSendMessageUIClosedDelega-) & Delegate )